1. Field of the Invention
This invention relates to a method and a storage system for reducing contamination of a photomask. More particularly, this invention relates to a method and a storage system for reducing contamination of a photomask by varying a temperature of the photomask.
2. Description of the Related Art
Lithography technology is one of main technologies for semiconductor integrated circuit fabrication processes, and is used to transfer a predetermined circuit pattern on a photomask onto a wafer by virtue of a lithography etching process. As a result, any defect or haze contamination on the photomask can be transferred to the wafer during a lithography process, resulting in an adverse affect one yield rate of subsequent processes. In particular, with the increasing development of integrated circuit technologies, the electronic products made thereby have more and more diverse functions, and the photomask used in the related technologies have smaller size restrictions and more complicated patterns. Therefore, any small defects or contaminants on the photomask will adversely affect the products. As such, the quality of the photomask is one of the key factors in semiconductor fabrication.
The contamination or haze formation on the photomask may be attributed to micro-dust particles falling onto the photomask, or to contaminating gases (for example, H2SO4, H2S, SO2, NH4, etc.) that are deposited on the photomask after being subjected to excitation and reaction processes occurring during an exposure process. The contaminating gases may come from the materials that are used for making the photomask (such as, a substrate, a frame, an adhesive, etc.), residues of chemical substances (such as, sulfuric acid, hydrogen peroxide, ammonia, etc.) that are used in an etching process for forming a pattern on the photomask, a contaminating gas that exists in the environment, such as a storage system, or a clean room for the photomask, or a contaminating gas formed due to a scanning irradiation of the photomask.
In order to prevent micro-dust particles froth falling onto a circuit pattern of a photomask during transport or an exposure process, a pellicle is usually disposed on the photomask to cover the circuit pattern, by which the micro-dust particles can be kept outside of the depth of focus (DOF), and will not form any undesired image on a wafer. However, the contaminating gases are likely to accumulate between the photomask and the pellicle, after several times of exposure processes, and produce haze on the photomask.